It has been about 10 years since supersonic motors were first reported.
A supersonic motor has superior high torque characteristics. Practical use of supersonic motors is possible in various fields using these characteristics.
Up until now, however, no method has been established for accurately controlling the torque produced in the supersonic motor. That is, up until now, no one has proposed a method for independent control of the torque in a supersonic motor. As a result, supersonic motors have not been commercially used in applications for control of position and speed like those of electromagnetic type servo motors, for example, for as part of a robot arm for increasing or decreasing the speed of movement. That is, the supersonic motors up until now have not, unlike electromagnetic type servo motors, allowed the formation of current loops and therefore have not been able to be directly connected to and operated in cooperation with the general commercially available robot controllers.
Details will be provided below.
A supersonic motor is usually driven near the mechanical resonance point so as to reduce loss, but right from when supersonic motors were first reported, attempts have been made to control supersonic motors by changing the sensitivity of the motors by deliberately shifting the mechanical resonance point. This technique, however, is equivalent to controlling the supersonic motor by changing the impedance of the motor, so inherently destroys the linearity of the control system of the motor. Accordingly, stable control of a supersonic motor cannot be achieved.
At the initial stage after the first reports of supersonic motors, several proposals were made on speed control for the motors. All of these methods control the speed by detecting the output voltage of an amplitude sensor built in part of the supersonic motor or the absolute value of the amplitude or phase difference of the current flowing to piezoelectric actuators and immediately changing the resonance frequency. These methods of control, however, do not independently control the torque, so are not suited to applications involving formation of a servo loop as with electromagnetic motors.
A recently proposed improved method of control of a supersonic motor involves referring the rotational speed of the rotor against data stored as a table in a memory of a controller to calculate in reverse the torque produced and then changing the reference speed value. This method does in the end control the drive voltage or the resonance frequency, but requires detection of the rotational speed of the rotor or involves indirect parameters derived from the computation using the data stored as a table in the memory, is susceptible to noise, and cannot really be said to directly control the torque, so is still wanting in terms of formation of a servo loop.
In this way, the methods of control of supersonic motors proposed and commercialized up until now have not be well suited to cooperative operation of a supersonic motor built in as part of a commercially available robot controller.